The largest uncertainty in the radiative forcing of climate change over the
industrial era is that due to aerosols, a substantial fraction of which is
the uncertainty associated with scattering and absorption of shortwave
(solar) radiation by anthropogenic aerosols in cloud-free conditions
(IPCC, 2001). Quantifying and reducing the uncertainty in aerosol influences on
climate is critical to understanding climate change over the industrial
period and to improving predictions of future climate change for assumed
emission scenarios. Measurements of aerosol properties during major field
campaigns in several regions of the globe during the past decade are
contributing to an enhanced understanding of atmospheric aerosols and their
effects on light scattering and climate. The present study, which focuses on
three regions downwind of major urban/population centers (North Indian Ocean
(NIO) during INDOEX, the Northwest Pacific Ocean (NWP) during ACE-Asia, and
the Northwest Atlantic Ocean (NWA) during ICARTT), incorporates
understanding gained from field observations of aerosol distributions and
properties into calculations of perturbations in radiative fluxes due to
these aerosols. This study evaluates the current state of observations and
of two chemical transport models (STEM and MOZART). Measurements of burdens,
extinction optical depth (AOD), and direct radiative effect of aerosols
(DRE &ndash; change in radiative flux due to total aerosols) are used as
measurement-model check points to assess uncertainties. In-situ measured and
remotely sensed aerosol properties for each region (mixing state, mass
scattering efficiency, single scattering albedo, and angular scattering
properties and their dependences on relative humidity) are used as input
parameters to two radiative transfer models (GFDL and University of
Michigan) to constrain estimates of aerosol radiative effects, with
uncertainties in each step propagated through the analysis. Constraining the
radiative transfer calculations by observational inputs increases the clear-sky, 24-h averaged AOD
(34&plusmn;8%), top of atmosphere (TOA) DRE (32&plusmn;12%), and TOA
direct climate forcing of aerosols (DCF &ndash; change in radiative flux due to
anthropogenic aerosols) (37&plusmn;7%) relative to values obtained with
"a priori" parameterizations of aerosol loadings and properties (GFDL RTM). The
resulting constrained clear-sky TOA DCF is &minus;3.3&plusmn;0.47, &minus;14&plusmn;2.6, &minus;6.4&plusmn;2.1 Wm<sup>&minus;2</sup>
for the NIO, NWP, and NWA, respectively. With the use of constrained quantities
(extensive and intensive parameters) the calculated uncertainty in DCF was 25%
less than the "structural uncertainties" used in the IPCC-2001 global estimates
of direct aerosol climate forcing. Such comparisons with observations
and resultant reductions in uncertainties are essential for improving and
developing confidence in climate model calculations incorporating aerosol
forcing.